Fermiology, orbital order, orbital fluctuations, and Cooper pairing in iron-based superconductors

Abstract

We address two important issues that arise in recent studies of iron-based superconductivity. (1) Why are the ${T}_{c}$ of ${A}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ ($A=\mathrm{K}$, Rb, Cs) and the single unit cell FeSe on SrTiO${}_{3}$ so high despite both only having electron pockets? (2) What (if any) are the effects of orbital order and orbital fluctuation on the Cooper pairing. Our main conclusions are the following: (1) removing hole pockets releases frustration of Cooper pairing from their band vorticity, therefore can enhance ${T}_{c}$, and (2) orbital fluctuation has negligible effect on Cooper pairing.